Summary

The codebase already includes a ci-container.yml file, which builds a Docker image in CI on the pull_request event. However, these images are immediately lost upon being built. This PR indends to push these images to GitHub's container registry ghcr, such that PRs are more easily testable & deployable.

Additionally, this PR addresses the duplication of Docker build & push logic between cd-workflow.yml and ci-container.yml. This was already causing problems, as input build-args didn't match between the files and were missing the frontend google maps keys. Now, they are merged in a single workflow file, docker-build-push.yml, that is utilized in both workflows. I think this is the best way to go about doing this? Let me know.

• ci-container.yml creates a github token with write access and pushes the Docker image to ghcr with tags and labels that indicate its status as a PR
  • NOTE: is the arrangements of tags and especially labels desirable when laid out as they are here? I think the tags make sense, as it ties them to the PR and also allows a specific build to be referenced by hash.
• centralized Docker build & push login in a single reusable workflow. I followed these steps to extract the workflow.